why do semes always tell ukes they suck at blowjobs ╥﹏╥, it's so humiliating and pl...

isn't it normal they wouldn't be good at it if it's their first time?

I don't know, to me it's very rude and mean saying something like that to your parner even if it isn't an actual relationship and even if it's true (I'll never admit that my beloved innocent uke are not good at that (≧∀≦)), also seems like is uke fault from the way they always say it, some poor uke could even get a trauma, develop some complex.
P.S. that "it's not true" was ironic xD

In this story, the humiliation is the point, most likely. Kinoshita wants to maintain distance and show how he's above the whole thing. He obviously enjoyed it, but to admit that would change the dynamic between them. See how shocked he was when he got an erection from the last encounter.

It's the whole sadist/masochist interplay that's used often in BL manga.

And all the semes have that much experience knowing what a good blowjob is, huh? They have get used to the blowjobs that they find it now not good enough if a virgin mouth does it, huh? Sorry but I am not buying this.

Ps i like this manga!

If we're speaking about the stupid uke/ seme stereotypes, then yes, those semes probably have experience. Whenever the traditional uke/seme roles are used, the uke is mostly portrayed as innocent and without experience, while they portray the seme as having previous experience (mostly with women), to widen the gap between them.
(Also sidenote but those seme could have just watched a lot of porn lol, and have some expectations about the 'perfect blowjob'. Then when a lot of choking, coughing, gagging, whatever happens, it kind of makes sense they would say the other isn't good at it. They don't even need actual experience. (Of course this is not 'fair' by normal standards.))

However, the uke not being able to do it perfectly, doesn't mean the seme won't get aroused from it, because we all know, in all those manga, those cliched semes always enjoy it being done to them in the end. As the person before said, the humiliation is just to keep the power balance between them. Shortly summarized: the seme saying this sort of thing fits this type of story.

Now, if this was about an actual, loving relationship, and one partner bluntly told the other partner he/she sucked at giving blowjobs, then that would just be plain rude and mean and indeed possibly result in making the partner really unsure. But that ain't the case here. This uke feels happy he's even allowed to stay close enough to that seme to gíve him a blowjob. So you really shouldn't bring logical arguments of what a good relationship with a decent power balance should be like, into something like this. It's just yaoi. Porn.

I was only making fun of the fact that high school boys do not have that much of experience so they like every blowjob they can get except if the uke hurts them with his teeth. Actually i agree with you about this being a cliche and fits in this story.